1 Introduction

With the fast development and utilization of nuclear energy, large amounts of radionuclides are released into the natural environment, which not only have impacts on environmental pollution, but also threaten the human health through food chain. On the other hand, the sustainable development of nuclear power utilization, the extraction of uranium from spent fuel, wastewater or seawater is crucial to supply the nuclear fuel. In the last decade, many methods such as sorption, precipitation, photoreduction, electrochemistry precipitation, crystallization, chemical complexation, piezocatalytic, membrane separation, etc., have been used to separate the target radionuclides from complex systems (Chen et al. 2023a; Hao et al. 2023; Liu et al. 2023; Yang et al. 2021; Wen et al. 2023). However, the highly efficient selective extraction of target radionuclide is not only dependent on the properties of radionuclide itself, but also related to the properties of the materials such as structures, channel pore space, active sites, and special functional groups. Different kinds of materials such as polymers, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), graphene, etc., have been investigated and the results showed that the active sites, inner pore sizes, tunable structures and special functional groups are critical parameters to improve the high selective sorption of radionuclides (Gu et al. 2022; Leng et al. 2023; Sheng et al. 2017; Sun et al. 2018; Yang et al. 2023a; Yin et al. 2017). To improve the photocatalytic reduction of high valent radionuclide to low valence such as U(VI) to form U(IV) precipitate, the strategies to enhance visible light absorption and separation of photogenerated e-/h+ pairs are utilized. For the electrochemical enrichment of radionuclide, the active sites as electron transfer platform in the electrode, high conductivity and high stability of the electrode are most important parameters for separation of radionuclides (Chen et al. 2023b; Niu et al. 2023; Xie et al. 2023). Besides experiments, machine leaning can be firstly carried out to understand the interaction of radionuclides with nanomaterials, which is helpful to design and construct the nanomaterials with high performance (Wei et al. 2024a, b). Although many reviews have summarized the recent works about the sorption of radionuclides from solutions (Abney et al. 2017; Di et al. 2022; Mei et al. 2023; Wang et al. 2019; Xie et al. 2022a), the separation of cationic, anionic and gaseous radionuclides from complex systems using COFs by sorption, photocatalytic and electrocatalytic strategies are not systematically described. In this perspective, we mainly described the selective extraction of representative cationic radionuclide U(VI), anionic radionuclide Tc(VII) and gaseous radionuclide I2 by COFs through the abovementioned techniques, and challenges for real applications are summarized in the end.

2 Removal of radioactive uranium

235U(VI) is the most important element for nuclear fuel. The extraction of 235U(VI) from wastewater or seawater is thereby crucial for nuclear energy utilization. The commonly used methods for uranium extraction currently include adsorption, ion exchange, electrochemistry, solid-phase extraction, membrane filtration, biological treatment, photocatalysis, etc. (Wu et al. 2023; Zhang et al. 2023). In recent years, adsorption photocatalytic synergy has become a research hotspot. In addition, the adsorption sites of photocatalysts for U (VI) photoreduction reactions are the decisive factor in photocatalytic reduction of uranium. When U (VI) is adsorbed by the U (VI) adsorption site of the photocatalyst, the adsorbed U (VI) becomes the site for electron capture. As a result, the adsorbed U (VI) can easily obtain electrons and then be reduced. Therefore, constructing efficient U (VI) adsorption sites on photocatalysts is a necessary condition for developing advanced photocatalytic uranium reduction catalysts (Chen et al. 2022). COFs with special functional groups such as amidoxime group could form strong surface complexes with 235U(VI), to achieve the high selective sorption of 235U(VI) from solutions. Through tuning the local structure to enhance the visible light absorption, to generate/separate the e-/h+ pairs and to facilitate the charge transfer to adsorbed 235U(VI), the photoreduction of adsorbed U(VI) to U(IV) species could be enhanced and continuously formed precipitates on COFs. In this sorption-photocatalytic reduction strategy, the first step is the selective sorption of U(VI) by COFs, and then the second step is the reduction of U(VI) to form U(IV) precipitates (Chen et al. 2023b; Yang et al. 2023b). Through tuning the donor-acceptor (D-A) sites (Fig. 1a), the distribution of local charge and separation of charge carrier resulted in the enhancement of photocatalytic ability. The EPR analysis showed that the free active radicals of ·O2-, ·OH and 1O2 were generated under visible light irradiation. These oxygen-containing free radicals inhibit the growth of marine microorganisms, enabling COFs to exhibit excellent anti-biological pollution activity, thus ensuring efficient uranium extraction photocatalysis in seawater. The XAFS characterization showed the photocatalytic reduction of U(VI) to form UO2 precipitate. Through the advanced spectroscopy analysis, the reduction mechanism of U(VI) is illustrated in Fig. 1b. Furthermore, Feng et al. (2022) constructed hydrogen-bonded COFs using Cl- ions to connect the organic ligands. The electron-rich of COFs strengthened H-bonds and photogenerated electrons under visible light conditions could efficiently reduce adsorbed U(VI) photocatalytically to form U(IV) precipitate, with the capacity of 1.7 g U/g COFs. The results showed that the soluble U(VI) could be immobilized by the COFs, thereby reduced the dangerous of U(VI) in the environment.

Fig. 1
figure 1

a The strategy for the construction of D-A system in multivariate COFs. b Photocatalytic mechanism of U(VI) reduction to U(IV) by COFs under visible light irradiation (Yang et al. 2023b). c Adsorption kinetics of ReO4- by TFPM-PZ-Cl. d Removal efficiency of ReO4- by TFPM-PZ-Cl in the presence of competitive anions (Zhang et al. 2022). e Structure diagram and crystal structure (large spheres indicate pore size) (Guo et al. 2020). f Mixed-component (150 ppm I2 and 50 ppm CH3I) breakthrough experiments performed at 25 °C (Xie et al. 2022b). g Survey of Xe/Kr separation performance in top-performing MOFs, COFs, and porous organic cages (Jia et al. 2021)

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Up to now, most of the methods for uranium removal have revolved around the adsorption of the radionuclide uranium through the design of rational COFs structural sites. In addition, with the development of solar energy utilization technology, the intramolecular D-A COFs exhibit excellent U(VI) photocatalytic reduction properties. However, the COFs materials are usually in powder state, which is unfavorable for recovery and recycling, and can be considered to composite with other adsorption materials or made into electrode state. The electrocatalytic strategy for U(VI) extraction from seawater is also a very promising method for continuously deposition of U(VI) on the electrode with high selectivity and reusability. The combination of adsorption with photocatalytic and electrocatalytic methods for the removal of uranium deserves to be studied.

3 Removal of radioactive 99TcO4 - and ReO4 -

Anionic radionuclides such as 99TcO4- are most toxic radionuclides because of their high mobility, low sorption ability, high water solubility and low complexation property. 99TcO4- can easily diffuse into the natural environment, resulting in the radiotoxicity to human health through the accumulation in the food chain. Because of the high radiotoxicity, 99TcO4- is not allowed in most laboratories for conducting research as the license of 99TcO4-/ReO4- is normally used to understand the behavior of 99TcO4- because of their very similar physicochemical properties. Chen et al. (2023c) synthesized the ionic COFs and applied for the removal of 99TcO4- and ReO4- from solutions. The hydrophobic skeleton and high charge density of the ionic COFs lead to the high selective removal of 99TcO4-/ReO4-. Additionally, unlike 2D COFs, Zhang et al. (2022) constructed 3D COFs through Aldol condensation with TAMP-PZI or TFPM-PZI as cationic tetrahedral building block. The COFs exhibited high irradiation, strong acid and base stability, and batch adsorption experimental results have shown that TFPM-PZ-Cl can quantitatively remove ReO4- with a concentration of 28 ppm from the solution in almost 30 seconds, exhibiting high adsorption kinetics (Fig. 1c). Despite the presence of high concentrations of competing anions, TFPM-PZ-Cl remained good selectivity towards ReO4- (Fig. 1d). The positive ionic sites and the high porosity of the 3D COFs assured the selective adsorption of 99TcO4-/ReO4- with high efficiency. The DFT calculation further indicated the electrostatic repulsion between Cl- and 99TcO4-/ReO4- promoted the adsorption ability of 99TcO4-/ReO4-. In order to improve the stability of the adsorbent under extreme conditions, He et al. (2019) reported the separation of 99TcO4- under extreme conditions by 2D cationic COFs, and the results showed that the cationic COFs exhibited the sorption capacity of 702 mg/g ReO4-, with fast sorption kinetics, high sorption capacity and excellent selectivity through anion exchange of Cl- with 99TcO4-.

At present, various methods such as ion exchange, extraction, and precipitation have been used to capture 99TcO4-. Among them, the removal of 99TcO4- through ion exchange is the most widely used due to its ease of implementation and efficient recovery rate. COFs benefit from their ordered pore structure and large specific surface area, and their adsorption performance can exceed that of polymer materials without crystallinity. However, it is a great challenge to synthesize cationic COFs, which can be used stably for 99TcO4- removal. Therefore, it is crucial for practical applications to design the construction units of COFs reasonably and ensure their chemical stability under strong acid, strong alkali, and high radiation conditions.

4 Removal of radioactive iodine

Radioiodine (131I, 129I) is one most important radionuclide. In nuclear power plants, the iodine (I2) or iodomethane (CH3I) is forms and released from off-gas steam, which is easily volatilized and thereby diffuses to the environment, causing harm to ecological systems and human health (Xie et al. 2024). Liu et al. (2022a) constructed phthalocyanine-based Cu-COFs for the separation of I2 and CH3I, and achieved the sorption capacities of 3.0 g/g for I2 (T = 353 K, ambient pressure, contact time is 48 h) and 492 mg/g for CH3I (T = 298 K, contact time is 24 h). The outstanding ability was attributed to charge transfer from N-rich phthalocyanine and e--rich π-conjugated structures, which formed strong interaction with I2 molecule. I2 obtained charge to form polyiodide (Ix-), which further formed strong complexes with Cu(II) centres through electrostatic interaction, also resulting in the high sorption of I2. Besides the active sites and functional groups to transfer charges from COFs to I2 molecule to form strong electrostatic complexes, the micro-/macro-pores of COFs are also crucial for the separation of I2 or CH3I from solutions through the pore space effect. Hao et al. (2024) synthesized COFs using symmetric building blocks to generate tetragonal or hexagonal pores in COFs using imine-based linkers, and then partitioned the COFs with 2,5-diaminobenzonitrile (C2) or 5''-(4'-amino[1,1'-biphenyl]-4-yl) (C3) to divide the mesopore/micropore into two or three micropores. After partition, the COFs have more N-containing groups which are favorable to improve I2 adsorption performance. The batch experimental results showed that the capture of I2 and CH3I at 75 °C by the C3 divided COF (COF 3-2P) was much higher than that by the undivided COF (COF 3). The Raman, XPS and FTIR characterization illustrated that I2 was adsorbed in the forms of I5- and I3- through the interaction with the N-rich sites. After I2 adsorption, the electrons were transferred from N atoms to I2 to form I5- and I3-. The DFT calculation revealed that the adsorption of I2 or CH3I was mainly attributed to the interaction with the triazine N sites. The column experiments showed that I2 and CH3I were efficiently separated by COF 3-2P under different conditions. The breakthrough curves of I2 and CH3I in the COF 3-2P column were affected by the temperature obviously, and the adsorption of I2 and CH3I were affected by the temperature. The pore space of COFs could be partitioned into uniformly micropore size, which not only enhance the adsorption of I2 or CH3I molecules by adjusting the pore space of the COFs, but also separate them through controlling the pore size using suitable symmetric agents. This work firstly reported the partition of COFs using symmetric building blocks to change the inner-pore space and to introduce more N-rich sites, which were suitable to adsorb I2 molecules in the inner-pore spaces with the help of N-rich groups to improve I2 adsorption. Similarly, the adsorption of I2 by cage-derived COFs also indicated that the binding interaction of I5- and I3- with N-rich cage contributed the high adsorption of I2 (T = 348 K, contact time is 50 h) (Liu et al. 2022b). In order to further improve performance and structural stability, Guo et al. (2020) synthesized colyliform 2D COFs with quasi-3D topologies, large pore spaces and flexible constitution units (Fig. 1e). The COFs exhibited high adsorption capacity of I2 (6.29 g/g) at 75 °C and 1 bar with high irradiation stability and reusability. The V-shaped monomer in the COFs not only increased the adaptive ability and flexibility of COFs, but also reduced the π-π interaction of COF interlayers, thereby enhanced I2 adsorption. Adsorbents that can effectively remove I2/CH3I are being increasingly studied, but few adsorbents can effectively adsorb low concentrations of I2 and CH3I simultaneously. Xie et al. (2022b) reported the simultaneous adsorption of I2 and CH3I by COF-TAPT. In the dynamic mixed-gas adsorption with 150 ppm of I2 and 50 ppm of CH3I, COF-TAPT presents an excellent total iodine capture capacity (1.51 g·g-1 at 25 °C), surpassing various benchmark adsorbents (Fig. 1f). The simultaneous capture of I2 and CH3I was attributed to the intermolecular interaction with I2 through the phenyl rings, triazine and imine moieties, and the N-methylation reactions at the nucleophilic N sites to promote CH3I capture.

Currently, the adsorption of radioactive iodine (volatile iodine and organic iodine) by COFs is limited to the laboratory stage. It is possible to simulate the real environment where radioactive iodine is located under the premise of guaranteeing the safety, to determine the selectivity and adsorption performance of COFs under real and complex conditions, and to evaluate their stability. Thereby, it is necessary to provide more persuasive research data for the practical application of radioactive iodine treatment.

5 Removal of other radioactive nuclides

Thorium is also one of the naturally occurring radioactive elements. In nature, the content of thorium is four times that of uranium. Thorium is considered the ‘nuclear fuel of the future’ because of its physical and chemical properties that are superior to those of uranium, its higher energy density and the fact that 232Th can be converted to 233U by neutron bombardment in nuclear reactors (Jyothi et al. 2023). A three-dimensional COF material named COF-DL229 has been successfully synthesized and applied to effectively selective entrapment of Th (IV) (Liu et al. 2022c). Due to the exposure of nitrogen-containing imine bonds, the maximum saturated adsorption capacity of this COF for Th (IV) can reach 513 mg g-1, with a fast capture rate and reaching equilibrium within 1 minute. The adsorption mechanism was further explored through DFT calculations. The DOS results indicates that the addition of Th (IV) can reduce the band gap of COF-DL229, improve both conductivity and adsorption energy. Therefore, this work provides scientific research value for using three-dimensional COF materials in the field of Th (IV) adsorption. Compared to the solid and liquid fission products generated by nuclear reactors, the safe disposal of gaseous fission products is more difficult. Jia et al. (2021) prepared two novel sub-nanoporous covalent organic frameworks (COFs) using a multiple-site alkylation strategy and applied them for the separation of xenon/krypton for the first time. Due to their excellent stability, designability of structure, and slightly larger pore size (~7 Å) than the dynamic diameter of krypton/xenon, the COFs realize the effective adsorption and sieving of krypton/xenon. In comparison with previously reported metal organic frameworks (MOFs) and porous organic cages, TFP-TAPA-Bu exhibits outstanding adsorption and separation performance for Xe/Kr (Fig. 1g).

In summary, existing work reports have demonstrated the enormous potential of COFs in spent fuel reprocessing and nuclear environment remediation, providing important theoretical and experimental basis for their practical applications.

6 Perspective

From the abovementioned representative works about the synthesis and application of different types of COFs for the separation or extraction of radionuclides uranium, iodine and technetium from complex systems, one can conclude that COFs exhibited high selectivity and sorption capacity to adsorb the target radionuclides from complex solutions under different conditions, even from strong acidic or basic solutions. The modification of active sites and stable active centres, layer stacking mode, inner pore size and space configuration, adaptive and flexibility of structures, functional groups, and electron transfer pathway could provide special properties of COFs, which are favorable for the binding of target radionuclide with high selectivity, ability, stability, and reusability through ion exchange, surface complexation, photocatalytic reduction and electrocatalytic precipitation strategies. However, for the real application of COFs in the extraction of radionuclides from wastewater, there are still several challenges. For example, 1) the synthesis of COFs is generally carried out under relative complicated conditions. The mild condition for the synthesis of COFs should be developed; 2) the construction of COFs is generally dependent on the experimental experiences. The accurate design of COFs is really a little difficult without abundant background knowledge. Machine learning could help us to understand, to predict and to select the most suitable precursors for COFs construction; 3) the stability of COFs is usually under very extreme conditions, especially in nuclear spent fuel conditions such as strong irradiation, strong acid and presence of different homologue radionuclides. The very similar physicochemical properties of the homologues make them difficult to be separated efficiently; 4) the synthesis of COFs in large scale at low price is the main challenge for real application. With the development of technology and the experience of COFs synthesis, COFs could be synthesized easily at low cost in large scale. The grafting of special groups to bind the target radionuclides or the post-modification of COFs with tunable pore space and structure could guarantee the special separation of radionuclides under extreme conditions. Thereby, we could conclude that COFs could be the promising materials for the radioactive wastewater treatment in future.